In general, a polymer electrolyte membrane fuel cell (PEMFC) has been widely used as a fuel cell for a vehicle. When a stack which is manufactured by laminating several hundreds of unit cells of the PEMFC is loaded on the vehicle, the unit cells of the fuel cell should stably operate over a wide current density range such that the stack normally exhibits a high-power performance of at least several tens of kW under various operational conditions of the vehicle.
In reaction mechanism of the fuel cell for generating electricity, after hydrogen supplied to an anode as an oxidation electrode in a membrane electrode assembly (MEA) of the fuel cell is dissociated into hydrogen ions (protons) and electrons, the hydrogen ions are transmitted to a cathode as a reduction electrode through a membrane, and the electrons are transmitted to the cathode through an external circuit. At the cathode, as the hydrogen ions and electrons react with oxygen molecules together, electricity and heat are produced and, at the same time, water is produced as a reaction by-product. When an appropriate amount of water produced during the electrochemical reaction is present in the fuel cell, the humidity of the MEA is desirably maintained. However, when an excessive amount of water is produced and is not appropriately removed, a “flooding” phenomenon occurs, particularly at high current density, and the flooded water prevents the reactant gases from being efficiently supplied into cells of the fuel cell, thereby causing the voltage loss.
As described above, because water is produced in the PEMFC by electrochemical reaction of hydrogen with oxygen in the air, electrochemical performance and durability may be reduced due to physical damage on cell parts such as MEA and gas diffusion layer, and on the interface between parts of fuel cells, when freeze and thaw cycles are repeated within the range from sub-zero temperature to room temperature or higher. Accordingly, robustness of the electrode in the MEA is important in order to increase freeze/thaw durability in the fuel cell. Particularly, during freezing and thawing cycles, the electrode should not be deformed or fractured even under ice forming and growing conditions in the MEA, and it is necessary that the electrode is well attached to the membrane of the MEA and, at the same time, electrode catalysts are well combined together.
Meanwhile, the fuel cell also needs to humidify the membrane in the MEA to provide water thereto by using an external humidifier. Further, dehumidification or dry-out repeatedly occurs because the cell is generally operated at high temperature of about 60° C. or greater and heat is generated itself by the electrochemical reaction, and thus, dry-wet cycling of the MEA is repeated. Under this dry-wet cycling condition, wrinkle deformation may be generated in the MEA. Accordingly, robustness of the electrode attached to the membrane as well as robustness of the membrane is important to well maintain the durability of the MEA for fuel cells.
However, the freeze and thaw durability or the dry-wet cycling durability of the MEA, which is important to the fuel cell, generally requires evaluation period of at least several months. Thus, structural factors for developing and commercializing MEA with improved durability may not be deduced in a short period of time.
Accordingly, it is necessary to quantify mechanical properties of the electrode itself, which are closely related to the long-term durability of the MEA. However, because the electrode itself may be easily damaged or deformed even by minor external stress or impact, separating only the electrode without said damage or deformation has not been successfully conducted. Typically, an electrode for MEA may be manufactured by coating a catalyst ink on a decal transfer film. The catalyst ink may be generally prepared by mixing platinum catalyst supported on (Pt/C), and ionomer binder with solvent mixtures mainly composed of isopropyl alcohol and deionized water, and then coated on the decal transfer film followed by drying thereof to manufacture an electrode.
In the related arts, measuring the mechanical properties of the electrode as being attached to the MEA has been attempted by using Double Cantilever Beam (DCB) method. However, such method may be disadvantageous because the unique properties of the electrode may be deformed by high temperature (e.g., about 100° C. or greater) and physical pressure applied when transferring the electrode coated on the decal transfer film onto the membrane. Furthermore, the expensive MEA may be used and wasted by being subjected to a fracture test. Accordingly, when the mechanical properties of the electrode is quantified as being separated from the decal transfer film, not as being tested from a final MEA product, unique properties of the electrode may be readily understood, the expensive MEA may not be wasted by a fracture test, and efficiency of developing process of a robust MEA may be increased by screening defective electrodes for MEA samples in advance prior to actually testing the long-term durability of the MEA samples.
However, so far, such method has not been reported to quantify the mechanical properties of the electrode, before transferring the electrode onto the membrane, by separating the pristine electrode from the decal transfer film on which the electrode is coated.
As conventional methods for separating a stacking member of a plurality of materials, a variety of cooling methods, for example, a separation method by spraying solid particles such as ice between joined layers, a separation method using swelling, caused by freezing liquid injected into micropores in between a substrate and a crystal thin layer, to separate the crystal thin layer grown on the substrate, a separation method by cooling a separating layer with ice in a structure of semiconductor substrate, a separating layer and a semiconductor layer, so as to reduce strength of the separating layer due to stress caused by swelling and shrinking, and a method for peeling a donor substrate of an assembly by cooling to the temperature lower than the room temperature have been suggested.
However, these conventional layer separation methods may have problems such that a subject to be separated during the layer separation process may be damaged, or the layer may be not completely separated. Accordingly, those methods may not be used for separating the pristine electrode for a fuel cell MEA, and particularly, for completely separating the pristine electrode for a fuel cell MEA for quantification of its mechanical properties due to damages during the separation process.
The description provided above as a related art of the present invention is just for helping in understanding the background of the present invention and should not be construed as being included in the related art known by those skilled in the art.